Rapid fabrication of uniform nanopores and nanopore arrays in Si3N4 membranes (see figure) is achieved using a high-intensity electron beam. Expansion or contraction of the pores under electron-beam irradiation is observed and utilized for the controlled fabrication of highly uniform nanopores in the size range 2-20 nm. Our nanopores exhibit a truncated double-cone structure, as revealed by 3D-TEM tomography.